This invention relates to a method for bonding a dry fiber strip to a dry fiber strand and, in general, to the production of fiber-reinforced components made of plastic, in particular for aviation.
FIG. 1 shows the layered construction of profile/stringers made of fiber composite materials (e.g., CRP, GRP, aramid-fiber-reinforced plastic, or combinations of these materials), which requires the use of gussets 2 for aviation applications to form an empty space in T-joints. This gusset 2 has a three-dimensional, typically triangular, and commonly, equilaterally triangular, cross-section.
To produce such a component, the gusset 2 is first applied to the base layer 1 of the component. The invention relates to this bonding of the gusset and the base layer as a partial process in the production method of the overall component. The base layer implemented may be a fiber scrim or fiber fabric, for example.
A tracer thread T is braided into the gusset during its production for unique positioning of the three edges of the gusset 2. This thread identifies the edge of the gusset that points away from the base layer.
Because of the production process of the gusset (braiding of individual thin threads, which themselves in turn comprise parallel or twisted rovings), the gusset may be twisted because of manufacturing, so that it must be untwisted before the processing.
The connection of the gusset and the base layer is currently performed manually. The gusset is cut to the required length from a supply roll in accordance with the dimensions of the component in which the gusset is to be processed. One end of the twisted gusset is fixed on a positioning tool having a triangular groove, so that the tracer thread points into the groove. The gusset is then untwisted manually. Its other end is also subsequently fixed on the positioning tool.
In the next processing step, the gusset and the base layer are bonded. For this purpose, the base layer is laid on the positioning tool. Because the gusset and the base layer are dry components, which are thus not already impregnated using a resin, a bonding agent, e.g., a thermoplastic binder nonwoven, is used to bond the two elements. Other adhesives or binder variants, such as powdered binders, may also be used for fixing the elements, however. One side of the base layer is provided with such a thin binder nonwoven. The gusset is fixed to the base layer by introducing heat, for example by using a heated metal plate (“clothes iron”). The binder nonwoven located on the base layer melts due to the effect of heat and glues the gusset after it cools.
A method for producing a semifinished product made of multiple mats or strips in continuous, uninterrupted operation is described in German document DE 698 14 129 T2. The mats or strips to be bonded are transported via rolls. The bonding is performed by heating and pressure impingement of a bonding agent using heated plate and pressure slides.
German document DE 2 201 334 A describes a method for laminating individual film layers as warps and wefts. The film layers to be bonded are suctioned by a partial vacuum and bonded by electrostatic charge.
Methods for producing planar fibrous semifinished products are known from U.S. Pat. Nos. 3,726,751 A and 4,460,633 A, and from German document DE 103 016 646 A1, in which individual threads or fiber strands are bonded to one another while crossed.
This invention has, as one object, the object of providing a method by which the base layer and the gusset may be bonded more rapidly and cost-effectively. In particular, complex untwisting of the gusset is to be dispensed with.
This object may be achieved by a method for bonding a dry fiber strip to a single dry fiber strand, in an axial direction and in continuous, uninterrupted operation, by way of a bonding agent that is activatable by supplying heat provided on the fiber strip. The fiber strand has at least one flat mantle surface and a three-dimensional cross-section. The fiber strip and the fiber strand are unwound from respective supply rolls, and the fiber strip is fixed on a conveyor belt using suction. The flat mantle surface of the fiber strand is positioned on the fiber strip during the unwinding procedure. The fiber strip and the fiber strand positioned thereon are covered using an air-impermeable film in a suction area to improve the suction effect, and heat is supplied during the unwinding procedure using hot air blowers to cause activation of the binding agent and gluing of the fiber strip and the fiber strand. Advantageous embodiments of the invention are apparent from the subject matter of further claims.
The gusset is also referred to hereafter as a fiber strand, and has a formation like a cord or cable. The base layer is also referred to hereafter as a fiber strip. Because its width is significantly greater than its thickness, it has a planar formation. The width of the fiber strand is, typically, significantly less than the width of the fiber strip, providing a strand-to-strip width ratio of 1:5.
According to one method according to the invention, the bonding of a fiber strip and a fiber strand occurs in a continuous, automated process. The fiber strip and the fiber strand are continuously unwound from their supply rolls, positioned one top of another, and bonded to one another while supplying heat. The fiber strand and the fiber strip both run in a mutual direction, which corresponds to the direction of their main axes. In other words, the fiber strand and the fiber strip do not intersect, but rather run parallel to one another. The bonding is achieved using a bonding agent activatable by supplying heat, which is located on the fiber strip. The heat is supplied using hot air blowers. The bonding agent may be a thermoplastic binder nonwoven, for example. However, other adhesives or binder variants such as powdered binders may also be used.
It is no longer necessary to untwist the fiber strand, because the fiber strand is now processed directly and continuously down from the supply roll (where it is provided in the untwisted state).
The invention has the particular advantages of shortening manufacturing times, making products reproducible, and permitting optimization of positioning tolerances.
According to the invention, the fiber strand and the fiber strip are transported using a conveyor belt, on which the fiber strip and the fiber strand are laid one on top of another. The conveyor belt is air-permeable so that the fiber strand may be fixed on the conveyor belt by suction.
According to the invention, the fiber strip and the fiber strand positioned thereon are covered by an air-impermeable film in the suction area. This improves the suction effect, so that the fiber strip is securely fixed on the conveyor belt. The film clings closely to the external surface of the overall body formed by fiber strip and fiber strand. This effect allows reliable fixing of the fiber strand on the fiber strip, because the fiber strand no longer has lateral movement capability.
An additional measure for positioning and fixing the fiber strand on the fiber strip is to use a mold matrix which forces the fiber strand into the intended position upon passage. The mold matrix may also be heatable for activating the bonding agent (e.g., melting of a binder nonwoven located on the fiber strip).
The advantage of suction with air-impermeable film is that no warpage may occur within the fiber strand during transport. In addition, positioning occurs without wear (no abrasion on components). Moreover, the film may also be made transparent without further measures, so that an improved capability for quality control results, because the correct position of the tracer thread may then be visually monitored.
Overall, the invention combines a heat supply for adhesive activation using a blower, transport on a conveyor belt, positioning/fixing of the fiber strip and the fiber strand by partial vacuum, and coverage of the fiber strip and the fiber strand by an air-impermeable film. This combination results in a very low-contact method, which allows very careful processing of the materials and largely avoids material warpage.
The film may be configured as an endless film, which is unwound from a supply roll before the area for supplying heat and received on a rollup roll passing the area for supplying heat.
The fiber strip and the fiber strand may be made from typical fiber materials, such as carbon fibers, glass fibers, or aramid fibers. The fiber strip and the fiber strand may also include different fiber materials.
Embodiments of the invention are explained in greater detail hereafter with reference to the drawings.